Method of producing nanocrystalline monolithic articles

ABSTRACT

A products having at least a portion thereof with a nanocrystalline microstructure, and methods of producing such products. The method generally entails machining a body to produce a polycrystalline chip having a nanocrystalline microstructure. The chips produced by the machining operation may be in the form of particulates, ribbons, wires, filaments and/or platelets. The chips may be consolidated (with or without comminution) to form a product, such that the product is essentially a nanocrystalline monolithic material consisting essentially or entirely of nano-crystals, or of grains grown from nano-crystals. Alternatively, the chips may be dispersed in a matrix material, such that the product is a composite material in which the chips are dispersed as a reinforcement material. According to a particular aspect, a monolithic article can be formed entirely from a single chip by deforming the chip and/or removing material from the chip.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. ProvisionalApplication No. 60/677,248, filed May 3, 2005, and is acontinuation-in-part patent application of co-pending U.S. patentapplication Ser. No. 10/707,999, filed Jan. 30, 2004, which is adivision application of U.S. Pat. No. 6,706,324, which claimed thebenefit of U.S. Provisional Application No. 60/244,087, filed Oct. 28,2000. The contents of these prior applications are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to methods of producingmaterials and articles with nanocrystalline microstructures, and moreparticularly to producing such materials and articles by machining andthen using the nanocrystalline material to form a product.

Significant benefits can be gained by deforming metal alloys through theapplication of very large plastic strains. Principal among these aremicrostructure refinement and enhanced mechanical and physicalproperties. Of particular current interest is the use of “severe”plastic deformation (SPD) to produce bulk solids with ultra-fine grainedmicrostructures (UFG's), especially nanocrystalline structures (NS)characterized by their atoms arranged in crystals with a nominaldimension of less than one micrometer. Nanocrystalline solids havebecome of interest because they appear to have significant ductility,formability and resistance to crack propagation, and possess interestingchemical, optical, magnetic and electrical properties. Nanocrystallinesolids also appear to respond to radiation and mechanical stress quitedifferently than microcrystalline materials (crystals with a nominaldimension of one micrometer to less than one millimeter), and theirresponse can be varied by changing the crystal size. Materials made byconsolidating nanocrystalline powders have also been shown to haveenhanced attributes not typically found in conventional materials. As aresult, nanocrystalline materials are believed to have significantpotential for use in industrial applications, provided they can bemanufactured in a cost-effective manner.

Multi-stage deformation processing is one of the most widely usedexperimental approaches to studying microstructural changes produced byvery large strain deformation. Notable examples include such techniquesas rolling, drawing and equal channel angular extrusion (ECAE). In thisapproach, very large plastic strains (true plastic strains of four ormore) are imposed in a specimen by the cumulative application ofdeformation in multiple stages, the effective strain in each stage ofdeformation being on the order of one. The formation of micro- andnanocrystalline structures has been demonstrated in a variety of ductilemetals and alloys using multi-stage deformation processing. However,there are significant limitations and disadvantages with this processingtechnique. A significant limitation is the inability to induce largestrains in very strong materials, such as tool steels. Other limitationsinclude the inability to impose a strain of much greater than one in asingle stage of deformation, the considerable uncertainty of thedeformation field, and the minimal control over the important variablesof the deformation field—such as strain, temperature, strain rate andphase transformations—that are expected to have a major influence on theevolution of microstructure and material properties.

The most widely used technique for synthesizing nanocrystalline metalshas been by condensation of metal atoms from the vapor phase. In thistechnique, the metal is evaporated by heating and the evaporated atomsthen cooled by exposure to an inert gas such as helium or argon toprevent chemical reactions, thereby enabling the purity of the metal tobe maintained. The cooled atoms condense into single-crystal clusterswith sizes typically in the range of 1 to 200 nm. The production ofceramic nanocrystals is similar, except that evaporated metal atoms aremade to react with an appropriate gas, e.g., oxygen in the case of oxideceramics, before they are allowed to condense. The resulting crystalsmay be compacted and sintered to form an article, often at a sinteringtemperature lower than that required for a microcrystalline powder ofthe same material. While suitable for making powders and small compactedsamples with excellent control over particle size, the condensationmethod is at present not practical for most applications other thanexperimental. A particularly limiting aspect of the condensation methodis the inability to form nanocrystalline materials of alloys because ofthe difficulty of controlling the composition of the material from thevapor phase. Another limiting aspect of the condensation method is thathigh green densities are much harder to achieve as a result of thenano-size particles produced. Other methods that have been explored tosynthesize nanocrystals include aerosol, sol-gel, high-energyball-milling, and hydrothermal processes. However, these techniquescannot produce nanocrystalline materials at a cost acceptable forpractical applications.

From the above, it can be seen that it would be desirable if a morecontrollable and preferably low-cost approach were available forsynthesizing nanocrystalline solids for use in the manufacture ofproducts. It would be desirable if such an approach were capable ofproducing nanocrystalline solids of a wide variety of materials,including very hard materials and alloys that are difficult orimpossible to process using prior art techniques.

In addition to the above-noted efforts directed to producing materialsthrough consolidation of nanocrystalline powders, there is also muchinterest in the creation of micro- and meso-scale parts such as shafts,disks, and gears, from functional materials such as metals, alloys andceramics. As used herein, micro- and meso-scale parts are two andthree-dimensional articles with feature sizes on the order of a fewmicrometers to a few millimeters. While the application domains forthese parts are not yet precisely demarcated, they are expected to befar and wide, encompassing ground transportation, biomedical,microelectromechanical systems (MEMS), aerospace, power generation,defense, nuclear industries, and others. Typical micro- and meso-scalemanufacturing processes currently being explored include materialremoval processes such as energy beam machining, micro-milling,micro-turning, laser ablation, and micro electro discharge machining(micro-EDM). These micro- and meso-scale processes employ subtractivematerial removal machining technologies and complement additive materialprocesses such as rapid prototyping and LIGA (an acronym derived fromthe German words lithographie, galvanoformung, and abformung(lithography, electroforming, and molding).

BRIEF SUMMARY OF THE INVENTION

The present invention provides products having at least a portionthereof with a nanocrystalline microstructure, and methods of producingsuch products. The present invention further provides a method by whichsmall-scale articles can be produced from UFG and especiallynanocrystalline materials. The latter aspect of this invention involveslow-cost methods of making nanostructured and UFG materials, combinedwith forming and/or material removal processes, to create small-scalearticles capable of exhibiting enhanced structural and mechanicalperformance.

The method generally entails machining a body to produce apolycrystalline chip having a nanocrystalline microstructure. Forexample, the body may be machined in a manner that produces chipsconsisting entirely of nano-crystals as a result of the machiningoperation being performed in a manner that imposes a sufficiently largestrain deformation. The body can be formed of a variety of materials,including metals, metal alloys, intermetallics, and ceramic materials.Furthermore, the body may have a microstructure that is essentially freeof nano-crystals, and may even have a single-crystal microstructure. Thechips produced by the machining operation may be in the form ofparticulates, ribbons, wires, filaments and/or platelets. The chips maybe consolidated (with or without comminution) to form a product, suchthat the product is essentially a nanocrystalline monolithic materialconsisting essentially or entirely of nano-crystals, or of grains grownfrom nano-crystals. Alternatively, the chips may be dispersed in amatrix material, such that the product is a composite material in whichthe chips are dispersed as a reinforcement material. According to aparticular aspect of this invention, a monolithic article can be formedentirely from a single chip by deforming the chip and/or removingmaterial from the chip.

The above features of the invention are based on the determination thatnanocrystalline structures can be formed in materials by machining underappropriate conditions to produce very large strain deformation,including high strain rates, such as a plastic strain of about 0.5 toabout 10 and a strain rate of up to 10⁶ per second. Machining processesbelieved to be capable of producing suitable nanocrystalline structuresinclude cutting and abrasion techniques. Cutting speed does not appearto be determinative, such that essentially any cutting speed can be usedif a cutting tool is used to perform the machining operation. Becausethe production method for the chips is a machining operation whoseparameters can be precisely controlled, the desired nanocrystallinemicrostructure for the chips can be accurately and repeatably obtainedfor a given body material. Furthermore, the machining operation can beadjusted to produce chips of various grain sizes and macroscopic shapesfor use in a variety of application. The production of nanocrystallinechips can often be achieved with this invention without having anynegative impact on the article being machined, such that nanocrystallinechips can be produced as a useful byproduct of an existing manufacturingoperation. As the byproduct of such a manufacturing operation, it isnotable that the chips produced and utilized by this invention wouldpreviously have been viewed as scrap produced by the operation, andtherefore simply discarded or melted for recycling.

In view of the above, the present invention provides a controllable andlow-cost method for synthesizing nanocrystalline solids that can be usedto produce monolithic and composite products. The method of thisinvention also makes possible the capability of producingnanocrystalline solids from materials that have been difficult orimpossible to process using prior art techniques, such as very hardmaterials that cannot be processed by multi-stage deformation processes,and alloys that cannot be processed by the condensation method.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically represents the process of machining a body with acutting tool to produce nanocrystalline chips in accordance with oneaspect of this invention.

FIG. 2 is a transmission electron microscopy (TEM) image of anelectron-transparent particle from a 52100 steel chip, and threeelectron diffraction patterns obtained from different regions of theparticle, which evidence that each of the corresponding regions in theimage is a distinct single crystal having a grain size of about 50 nm toabout 300 nm.

FIG. 3 is a graph showing the effect that annealing has on copper chipsproduced by machining to have nanocrystalline structures in accordancewith this invention.

FIG. 4 shows bright field TEM images of OFHC copper machined inaccordance with this invention to have nanocrystalline microstructuresresulting from different levels of shear strain.

FIG. 5 are TEM images showing the microstructures produced in anAI6061-T6 alloy as a result of machining at different strain levels inaccordance with this invention.

FIG. 6 show TEM images of the resulting grain growth (to about 200 nm)that occurred in two of the nanocrystalline microstructures of FIG. 5 asa result of thermal annealing for about one hour at about 175° C.

FIGS. 7 and 8 are TEM images of Inconel 718 and titanium foils,respectively, showing nanocrystalline structures after machining inaccordance with this invention.

FIG. 9 shows scanned images of nanocrystalline chips, and FIGS. 10through 12 represent micro- and meso-scale components of types that canbe machined directly from the chips of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

An investigation leading to this invention was undertaken to determinethe microstructure and mechanical properties of chips produced bymachining steels, and to explore the conditions under which they areproduced. Steel cylinders of AISI 52100, 4340 and M2 tool steel havingdiameters of about 15.7 mm were heat-treated by through-hardening andtempering to hardness values of about 60 to 62 R_(c), about 56 to 57R_(c), and about 60 to 62 R_(c), respectively. The initialmicrostructures of the steels prior to machining were temperedmartensite. The compositions (in weight percent), austenitizationtemperatures (A_(C3)) and approximate grain size (GS) of the steelspecimens are summarized in Table 1 below. TABLE 1 Steel Type (AISI)4340 52100 M2 C 0.38-0.43 1.00 0.8 Mn 0.6-0.8 0.31 0.35 Si 0.15-0.3 0.26 0.35 Cr 0.7-0.9 1.45 4.52 Ni 1.65-2.00 0.14 0.2 Mo 0.2-0.3 0.045.39 P 0.035 max 0.009 max 0.027 max S 0.040 max 0.019 max 0.005 max V —— 2.09 Cu — 0.09 0.13 Al — — 0.02 Co — — 0.39 W — — 6.86 Fe balancebalance balance A_(C3) (° C.) 815-845 775-800 1190-1230 GS (μm) 7 5   5Notably, M2 has a significantly higher austenitization temperature(A_(C3) of about 1200° C.) than the 52100 and 4340 steels (A_(C3) ofabout 800° C.). The cylinders were machined using a high precision lathewith polycrystalline cubic boron nitride (CBN) cutting tools. Undercertain conditions, including a cutting speed of about 50 to 200 m/min,a depth of cut of about 0.1 to 0.2 mm, and a tool feed rate of at least0.05 mm/rev, the so-called white layer (WL) was observed in the machinedsurfaces of the 51200 and 4340 steels, though not in any of the M2 steelspecimens. As known in the art, white layers are thin (typically lessthan 50 μm thick) hard layers that can form in the surfaces and chips ofcertain steels when subjected to machining, abrasion or sliding at highvelocities. White layers resist chemical etching and appear featurelessunder an optical microscope, hence the term “white layer.”

The chips produced under the above conditions were analyzed usingoptical microscopy, X-ray diffraction, transmission electron microscopy(TEM), and nano-indentation to establish their structure, compositionand mechanical properties. The nano-hardnesses of the chips wereestimated by making indentations of sub-micron depth using a Berkovitchindenter (three-faced diamond pyramid) on a nano-hardness tester(Nanoindenter XP). The indenter penetration depth was typically set atabout 200 nm, which typically corresponded to a load of about 10 mN.This penetration depth was substantially less than the size (on theorder of up to about 0.1 mm) of the chips being analyzed. Frommeasurements of the load-penetration curve during indentation andsubsequent unloading, both hardness and Young's modulus of the chipswere estimated. Electron transparent samples of particles taken fromchips were observed by TEM to determine their structures (e.g.,crystalline or amorphous) and grain size. For this purpose, the chipswere gently broken into smaller particles using a mortar and pestle,which were then separated by ultrasonic agitation in methanol. Theparticles were then placed on a grid and observed under the TEM. Some ofthe particles or regions of these particles were electron transparent,allowing direct images and diffraction patterns of these areas to beobtained. The diffraction patterns were used to establish thecrystallinity of the particles, while a combination of the bright fieldimage and diffraction was used to determine grain size.

At this point, it is worth noting that both the chips and the machinedsurface of each specimen were subjected to very large strain deformationduring machining. The deformation that occurs in the shear plane of achip can be seen in reference to FIG. 1, which represents the machiningof a workpiece surface with a wedge-shaped indenter (tool) as done inthe investigation. The material being removed by large straindeformation, namely, the chip, slides over the surface of the tool knownas the rake face. The angle between the rake face of the tool and thenormal to the work surface is known as the rake angle (α). The edge ofthe wedge penetrating the workpiece is the cutting edge. The amount ofinterference between the tool and the workpiece is the undeformed chipthickness depth of cut (t_(o)) and the relative velocity between thetool and the workpiece is the cutting velocity (V_(c)). When the toolcutting edge is perpendicular to the cutting velocity and the width ofcut is small compared to the cutting edge length and t_(o), a state ofplane strain deformation prevails, which is believed to be a preferredconfiguration for experimental and theoretical investigations ofmachining.

The chip formation in FIG. 1 is seen to occur by concentrated shearalong a plane called the shear plane, where a shear strain (γ) isimposed during chip formation. The shear strain can be estimated byEquation (1) below:γ=cos α/sin φ cos(φ−α)   (Eq. 1)where the shear plane angle (φ) is a known function of t_(o) and t_(c).The effective Von Mises strain (ε) can be predicted usingε=γ/(3)^(1/2)   (Eq. 2)Equation (1) shows that the shear strain (γ) can be varied over a widerange by varying the rake angle (α) from large positive to largenegative values (see FIG. 1). Additionally, the friction at thetool-chip interface also affects shear strain (γ) via its effect on theshear plane angle φ.

In view of the above, and as reported in the literature, effectiveplastic strains in the range about 0.5 to about 10 and strain rates ofup to 10⁶ per second can be generated with appropriate machiningconditions, as can a wide range of shear plane temperatures. Theseranges of values are substantially greater than can be realized intypical severe plastic deformation processes. Geometric parameters ofmachining like depth of cut (t_(o)) rake angle (α) and cutting velocity(V_(c)) affect the shear deformation in a manner analogous to the actionof dies in forging or extrusion. The effective plastic strain along theshear plane (deformation zone) in the chip can be systematically variedin the range of about 0.5 to about 10 by changing the tool rake angle,and to a lesser extent by changing the friction between tool and chip.The mean shear and normal stresses on the shear plane can be varied bychanging the tool geometric parameters together with process parameterssuch as V_(c) and t_(o), while the values of these stresses can beobtained from measurement of the forces. Finally, the temperature in thedeformation zone can be systematically varied by changing the cuttingvelocity. For example, by cutting at very low velocities (about 0.5mm/s), the temperature can be kept marginally above the ambienttemperature while achieving very large strain deformation.Alternatively, temperatures where phase transformations (e.g.,martensitic, melting) may be expected to occur in the chip can berealized by increasing the cutting velocity to higher values, forexample, about 1 to about 2 m/s. The ability to change the frictionalong the tool-chip interface by a factor of up to three has also beendemonstrated using a combination of tool coatings, low-frequencymodulation of the tool-chip interface and lubrication which assures thatlubricant is always present at the interface between the tool and thechip. The extent to which friction (as well as the other parameters andconditions discussed above) can be controlled in a machining operationis not possible in other severe plastic deformation processes. Insummary, the temperature, stress, strain, strain rate and velocityfields in the zone of deformation can be well estimated using availablemechanics models or obtained by direct measurement. Thus, very largestrain deformation conditions can be imposed and varied systematicallyover a wide range, a range over and beyond that currently obtainable inother severe plastic deformation processes.

In the investigation of the AISI 52100, 4340 and M2 steels, the normaland shear stresses imposed on the shear plane and on the work surface ofeach specimen was estimated to be about 2 to 4 GPa. The temperature riseof the chips was concluded to be typically greater than that of themachined surface, since the relatively large mass of the machinedsurface was capable of conducting away most of the frictional heatgenerated on the tool rake face as well as the heat generated in theprimary deformation zone (shear plane) due to plastic flow. The coolingrates in the chip and the workpiece were quite high, estimated as atleast 0.5×10⁵ ° C./s.

Table 2 summarizes the results of the nano-hardness measurements made inthe chips of the 52100 specimens and in the bulk of the 52100 specimens.TABLE 2 Hardness (GPa) Hardness Range (GPa) Young's Modulus (GPa) Chip12.85 ± 0.80 11.5-16.2 235 ± 8  Bulk 10.70 ± 0.85  9.0-12.0 235 ± 16The chip hardness value can be seen to be about 25% higher than, andstatistically distinguishable from, the nano-hardness value obtained forthe bulk 52100 steel. Furthermore, the chip hardness is significantlyhigher than Knoop hardness values reported for untempered martensiteproduced by quenching 52100 steel specimens. This difference issignificant, even accounting for the fact that hardness measurementsmade at sub-micron penetration depths generally yield slightly higherhardness values than Knoop hardness measurements. The Young's moduli ofthe chip and the bulk material, obtained by indentation, were virtuallyindistinguishable from that of the bulk material and close to thecommonly quoted modulus value of 220 GPa for steels.

FIG. 2 is a TEM image of an electron-transparent particle from one ofthe 52100 steel chips. Also shown in FIG. 2 are three electrondiffraction patterns obtained from different regions of the particle.The diffraction patterns indicate that each of the corresponding regionsin the image is a distinct single crystal, indicating that the particleis polycrystalline. Furthermore, from the TEM image of each of thesingle crystal regions, it is seen that the crystal (or grain) size isin the range of about 50 nm to about 300 nm. Analysis of TEM images ofother chip particles from each of the 4340, 52100, and M2 steels alsoshowed the chips to be polycrystalline with a grain size typically inthe range of about 30 to about 300 nm. All of the chips produced undermachining conditions that include a cutting speed of about 100 to 200m/min, a depth of cut of about 0.1 to 0.2 mm, and a tool feed rate of atleast 0.05 mm/rev, were shown to contain nanocrystalline structures(NS). Furthermore, the nano-scale grain sizes of the chips wereconsiderably smaller than the five to seven micrometer grain sizesreported above for the heat-treated steel specimens prior to machining.Consequently, it was apparent that the tempered martensite initiallypresent in the steel specimens had undergone amodification/transformation during machining. This conclusion isconsistent also with the comparative nano-hardness results discussedabove for the chips and the bulks of the steel specimens.

In research subsequent to the above investigation, it was concluded thatthe formation of nanocrystalline structures in machining processesappears to be a general phenomenon that goes beyond steels and beyondmachining with wedge-shaped tools. For example, measurements of thenano-hardness of chips in 4340 steel produced by grinding showed thelayers to have hardness values of about 12 to about 13 GPa, which wassubstantially greater than the hardness of the initial pearlitemicrostructures of the specimens. Microstructure analysis showed thesechips to have characteristics very similar to those of the chipsdescribed above. A preliminary TEM analysis of chips produced bygrinding titanium, copper, single-crystal iron and M50 steel at a wheelvelocity of 33 m/s also showed the resulting chips to be composed ofnanocrystalline structures. In yet another investigation,nano-indentation of brass chips produced by machining showed theirhardnesses to be about 50 to about 75 percent greater than that of thebrass material prior to machining, suggesting that these chips too maybe composed of NS. These observations suggest that a very large straindeformation that occurs during material removal under appropriateconditions may be the principal driving force for NS formation.

Based on the above investigations, it was concluded that machining atcutting speeds of about 100 to about 200 m/min consistently generatednanocrystalline structures in 52100, 4340 and M2 steels if asufficiently large strain deformation occurred. It is useful to considerother the conditions that may influence the formation of ananocrystalline structure in steels, based on an analysis of theirformation in machining and grinding. During machining, the zone of NSformation is subject to large strain deformation, localized hightemperature, high quenching rates, and possibly an austenite-martensitephase transformation. Some of these conditions may influence thegeneration of the NS seen in this investigation. However, except forlarge strain deformation, these other conditions can be readily achievedin conventional heat-treatment of steels. For example, theaustenite-martensite transformation, induced by rapid quenching, is akey element in heat treatment. However, there are no reports of NS insteels modified by heat treatment. Hardness values of martensite formedby heat treatment are also substantially lower than those of NS producedby high-velocity deformation in similar types of steels. Theseobservations suggest that while temperature-time histories and phasetransformations may be important in determining the range of grainsizes, physical properties and hardness of the NS, the conditions thatgenerate very large strain deformation appear to be essential for NSformation. This surmise is reinforced by observations of NS in thenon-ferrous metals described above, none of which undergo a displacivetransformation. That large strain deformation can result in very finegrain sizes is demonstrated by observations of microstructure inmacroscopic metal samples subjected to such deformation.

In another investigation, chips were formed by machining oxygen-freehigh conductivity (OFHC) copper, commercially pure iron and 1018 steel.The initial grains sizes for the specimens were about 170, 55 and 70micrometers, respectively. The machining conditions used for thesematerials included a depth of cut of about 2.54 mm and a tool feed rateof about 0.21 mm/rev. For the copper specimens, a cutting speed of about28.8 m/minute was used, while a cutting speed of about 6.375 m/minutewas used for the iron and 1018 specimens. Typical thicknesses for theresulting chips were about 200 to about 1000 μm. The chips were preparedby metallographic polishing for subsequent hardness and microstructureanalysis. Hardness measurements were done using Vickers indentation soas to obtain accurate measurements of the bulk hardness of the chips,devoid of surface-related contributions. The indent size was kept to atleast five times smaller than the dimensions of the chip sample toensure accuracy in the hardness measurements. Metallographicallypolished chip samples were also etched for analysis of grain size, flowpatterns and the presence of sub-structures within grains. Observationsof these chips were made using SEM, AFM, TEM and optical microscopy.

A parallel series of studies was made on bulk samples of the materialsso as to have an assessment of their hardness and microstructure priorto machining. The bulk samples were generally in a work-hardened stateprior to machining because they had been produced by drawing orextrusion processes. For reference, the bulk as well as chip samples ofsome specimens were annealed and hardness measurements repeated so thata true measure of the hardness changes caused by machining could beobtained. For this purpose, a series of annealing experiments wereperformed on the copper, iron, and 1018 steel chips to obtain anunderstanding of the recrystallization behavior of these chip samples.The recrystallization experiments were performed in a controlled argonatmosphere furnace using different time-temperature cycles. Hardness andgrain-size analysis of these samples were performed in accordance withthe investigation described above for the 4340, 52100 and M2 steels.

Table 3 is a summary of the Vickers hardness measurements, from which itis evident that for both the copper and iron specimens, the hardness ofthe chips was substantially greater than that of the material prior tomachining as well as that of the annealed samples. TABLE 3 VickersHardness (kg/mm²) Processing Condition: Base Chip Annealed Base Copper93 143 67 Iron 125 290 86 1018 steel 230 300 209Hardness measurements made at different locations over the entire chipvolume showed that the hardness distribution was essentially uniform. Astudy of the hardness distribution in partially formed chips produced ina specially devised experiment showed that the hardness increasedsharply when going from the bulk material into the chip as the shearplane is traversed.

FIG. 3 shows the results of recrystallization experiments on some of thecopper chips, and evidences that there was a critical temperature ofabout 100° C. below which there was no degradation in hardness valueseven for annealing times of six hours. This suggests that the mechanicalproperties of the chips can be retained through certain thermalprocessing cycles. Preliminary results from annealing experiments oniron and 1018 steel have suggested a similar recrystallization behaviorat significantly higher temperatures. The hardness of the copper, ironand 1018 steel chips was shown to be retained at room temperature evenafter about one thousand hours, though small changes in themicrostructures of the copper chips were observed after extended periodsat room temperature.

Finally, the examination of optical microscope, AFM, SEM and TEM imagesof bulk workpiece materials, chips, and recrystallized chips evidencedthat, while the bulk specimens had relatively large grains prior tomachining (ranging from about 55 to about 170 micrometers), the onlystructures resolved in the chips were sub-structures with dimensions inthe range of about 100 to 500 nm. FIG. 4 shows bright field TEM imagesof OFHC copper machined in accordance with this invention to havenanocrystalline microstructures resulting from different levels of shearstrain (γ). The corresponding selected area diffraction patterns verifythe polycrystalline microstructures, which are equiaxed and have meangrain sizes of about 150 nm. Features typical of onset of dynamicrecrystallization are visible in the chip indicated as being producedwith a shear strain of about 13. FIG. 5 shows microstructures producedin an AI6061-T6 alloy as a result of machining at different strainlevels in accordance with this invention. Typical grain size at thehigher strain (5.2) is about 90 nm. FIG. 6 shows TEM images of theresulting grain growth (to about 200 nm) that occurred in two of thenanocrystalline microstructures of FIG. 5 as a result of thermalannealing for about one hour at about 175° C. Finally, FIGS. 7 and 8 areTEM images of Inconel 718 and titanium foils, respectively, showingnanocrystalline structures after machining in accordance with thisinvention. Typical grain sizes are about 70-100 nm.

The series of investigations described above suggested the existence ofthe conditions under which nanocrystalline structures can be formed byvery large strain deformation in other ferrous and nonferrous metals andalloys, such as titanium, aluminum, tungsten and their alloys. Forexample, subsequent to the above investigations, nanocrystallinestructures with grain sizes of about 100 to 300 nm were produced inannealed iron and single-crystal tungsten by very large straindeformation. It was therefore concluded that a variety of materials canbe machined at various cutting speeds to consistently produce chipshaving grain sizes of about 30 to 500 nm, and likely below 300 and 100nm, if machining conditions impose very large strain deformations.Appropriate machining conditions will typically differ for differentmaterials, but can be readily determined by experimentation and verifiedby the presence of nanocrystalline grains. Because of the ability toindependently control a number of different machining conditions, it maybe possible to tailor a machining operation to produce chips with adesired grain size range. During the investigations, it was alsoobserved that chips with NS can be produced in varying shapes and sizes.Typical chip forms that have or could be produced include platelets,particulates, ribbons, helixes, wires, and tangled coils, obtained bymachining with a tool. While the formation of the chip involves shearingof the work material to very large strains in the narrow zone of theshear plane, the longitudinal and cross-sectional shape of the chip iscontrolled primarily by the machining conditions (e.g., feed and depthof cut) and the geometrical parameters of the tool. For example, byusing grooves on the tool rake face, the chip can be curled into ahelix. Therefore, it is feasible to produce high-strength,nanocrystalline chips with a wide variety of macroscopic shapes. Suchchips can be consolidated (with or without comminution) and sintered toproduce a nanocrystalline monolithic article, or used as continuousand/or discontinuous reinforcements for a variety of matrix materials,including polymers, metals and ceramics. The possibility of makinglow-cost continuous nanocrystalline reinforcements of different shapesby machining in accordance with this invention provides for theengineering of new materials.

From the above investigations, it was also concluded that it is verylikely that ceramic and intermetallic materials can be machined bycutting or abrasion to form chips composed entirely of nanocrystallinestructures, provided sufficiently sharp tools are used to form thechips. Under such conditions, large-strain plastic flow can be expectedto occur, and was observed in ceramic chips in the form of stringy,ribbon-like chips produced by abrading MgO ceramic and soda-lime glass.These chips showed characteristics very similar to those formed byplastic cutting of ductile metals. High velocity deformation can beexpected to occur when cutting brittle solids such as ceramics andglasses with a sharp tool, because of the imposition of significanthydrostatic compression in the zone of chip formation, together with theoccurrence of high temperatures. It is believed that machining withsharp, single-crystal or poly-crystal diamond tools at high cuttingspeeds would achieve plastic cutting in brittle solids to create fineparticulate or platelet-like chips composed of NS, as would high speedgrinding with diamond abrasive wheels of fine grit sizes at velocitiesof 30 to 100 m/s. Under such conditions, the chips are likely to besubjected to intense deformation and high temperatures because of thehigh cutting speeds involved and the imposition of extremely highpressure and shear at the abrasive-chip interface.

While nanocrystalline structures have been previously shown to exhibitunique intrinsic property combinations, achieving these propertycombinations in bulk materials has been hampered by the high cost ofnanocrystalline structures, i.e., those produced by condensationmethods. As discussed previously, nanocrystalline structures produced bycondensation methods are also essentially limited to nano-sizedparticles of pure metals or ceramics, with the ultra-fine particle sizeslimiting the green densities that can be achieved by consolidation.However, the present invention makes possible a source ofpolycrystalline materials with nano-sized grains, and can be produced atrelatively low cost, particularly since the invention has evidenced thatchips with nanocrystalline structures can be produced withoutcompromising the quality of the machined workpiece. As a result, thepresent invention makes possible a wide range of applications forlow-cost monolithic and composite materials containing nanocrystallinestructures. Comminution, (e.g., ball, attrition, jet milling, etc.) ofchips with nanocrystalline structures enables large-scale production ofpolycrystalline particulates that can be consolidated and densified intobulk monolithic materials more readily than nano-sized particlesproduced by condensation. Densification during sintering can be furtherpromoted because nanocrystalline materials often exhibit enhancedsintering kinetics due to high concentrations of crystal defects,allowing for the use of lower sintering temperatures. Though thermalprocessing may lead to coarsening (e.g., recrystallization and graingrowth, as shown in FIG. 6), the benefits of enhanced processing (lowersintering temperatures, higher densities, etc.) may outweigh thecoarsening effects in monolithic materials for numerous applications.The defect structures of nanocrystalline particulate produced bymachining may also enable new deformation processing routes tomonolithic materials (e.g., metal injection molding or tape casting),even for intrinsically brittle ceramics such as silicon nitride. Inaddition, special magnetic properties may be achieved in electricalsheet steel produced from nanocrystalline chips, such as improvedperformance in transformer cores.

Continuous ribbons and wires of high-strength nanocrystalline chips canbe used as reinforcement in composite articles and structures, includingconcrete for runways, highways and tunnels that are currently reinforcedwith steel wires. Alternatively, long chips can be broken to producespecific shapes and sizes of reinforcements. For example, platelets thatcan be produced by controlled breakage of ribbon chips may provide themost efficient stiffening of all discontinuous reinforcement shapes. Apreliminary investigation was conducted in which nanocrystalline chipsof M2 steel were broken to form platelets that were then incorporated ina bronze matrix by spontaneous (pressureless) melt infiltration, withoutapparent chemical interaction. Infiltration of an aluminum alloy (AI356) matrix around identically-formed M2 chips was also successfullyachieved. Characterization of the mechanical properties or the AI-M2composite was done by making sixty nano-indents at various locations inthe composite, from which the hardness and Young's modulus histogramswere obtained. Three different property clusters corresponding toindents lying in the AI matrix, the M2 reinforcement and the AI-M2interface region were identified in the hardness and Young's modulushistograms. The hardness and modulus values were, respectively, 0.8-1GPa and 70-90 GPa for the AI matrix, 1-4.5 GPa and 100-140 GPa in theinterface region, and 8-13 GPa and 180-235 GPa in the M2 steelreinforcements. Preliminary assessment of the mean hardness and modulusvalues for this composite has indicated that these are not significantlydifferent from those of a commercially available AI-SiC composite. Novisible cracking or reinforcement pull-out was observed aroundindentations made near the AI-M2 interfaces. These observations arepromising for the incorporation of nanocrystalline platelets, continuousribbons, filaments, and particulate reinforcements into a variety ofmetal matrices, including lightweight metal matrix materials such asaluminum and magnesium, which is of interest in a number of commercialground-transportation and aerospace applications, such as drive shafts,brake disks, and suspension components, where weight reduction iscritical. The use of metallic nanocrystalline reinforcements can beexpected to offer significantly improved wetting for liquid stateprocessing of conventional reinforcement materials, such as SiC.

Infiltration processing is just one of several feasible routes availableto produce composites using the NS reinforcements. Other processesinclude stir-casting, sintering and extrusion. One of the key issues isthe annealing behavior of nanocrystalline reinforcements duringcomposite processing. In this regard, secondary hardening tool andbearing steels such as M2 exhibit their peak hardening response in themelting range of aluminum and magnesium alloys, providing opportunitiesto improve reinforcement properties during processing of the composite.The incorporation of nanocrystalline chips in polymer matrices byimpregnation processing is viewed as being practical since polymercuring temperatures are relatively low, such that essentially noreinforcement annealing effects would be expected.

Finally, the present invention also encompasses direct creation ofarticles by above-described chip formation processes of this invention.In particular, the machining processes described above provide thecapability of directly producing chips of a variety of materials inplate-like shapes, or chips that can be deformed to have a plate-likeshape without loss of the desired nanocrystalline microstructure, andthen performing a post-machining operation by which a monolithic articleis entirely formed from a single chip, such as by deforming the chip orremoving material from the chip. FIG. 9 shows scanned photographicimages of the external morphology of tantalum and copper chips withmacro-scale dimensions produced directly by machining. The tantalum chipis approximately 75 mm long, 5 mm wide, and 500 micrometers thick. Thischip was produced directly by machining a tantalum block on aconventional shaper with a shaping tool, followed by roll straightening.The copper chip shown in FIG. 9 was also produced by shaping, but with asmall tension applied to the chip during its formation. The dimensionsof the copper chip are approximately 300 mm long, 5 mm wide, and 2 mmthick. The chips shown in FIG. 9 demonstrate the capability of creatinglarge polycrystalline directly by machining without any consolidation.Measurements of the deformed and undeformed thicknesses of the chipsshowed the shear strain in the chips during chip formation was well inexcess of two. Consistent with the previous discussion relating tostrain, under such large strain deformation conditions, there wassignificant refinement of the microstructure, resulting in grain sizeswithin the chips in a range of about 50 to about 1000 nm. The chips wereas much as three times harder than the bulk material from which theywere machined. Similar observations of microstructure refinement andassociated enhancements in mechanical properties were also made invarious other metals and alloys.

Based on the capability of producing such large nanocrystalline chips,it is believed that metallic nanocrystalline chips can be produced thatare several inches in length and width, with thicknesses of up to about0.25 inch (about 0.6 cm). Those skilled in the art will appreciate thatthe production of increasing large chips require the use of machineshaving sufficient power and appropriate tooling to accommodate suchlarge-scale chip formation. Additionally, the material being machinedshould be sufficiently ductile for the size of chip desired and themachine being used to produce the chip. Because macro-scale chipscreated by machining are likely to exhibit some degree of curvature,some type of post-machining deformation process, such asroll-straightening involving slight thickness reduction, may bedesirable to eliminate curvature. Additionally, it may be desirable toperform such post-machining deformation processes under differenttime-temperature conditions to impose additional control over themicrostructure and properties of the chip.

Post-machining thermal or thermo-mechanical processing of macro-scalechips are additional steps that can be performed for microstructurecontrol through recrystallization and recovery mechanisms. Since theresulting microstructure after thermal treatment is dependent on theprior strain imposed during chip formation and on the time-temperaturehistory of the thermal treatment, e.g., annealing, a variety offine-grained or UFG microstructures may be realizable with a high levelof homogeneity. The conditions of the machining, deformation, andthermal treatment processes can be varied to optimize the microstructurefor mechanical and physical properties of importance to any articleproduced from the chip.

Micro- and meso-scale articles can then be produced from the chip,effectively using the plate-like chip as a pre-cursor material orfeedstock tor manufacturing the micro- and meso-scale articles. Articlesformed in this manner preferably retain the nanocrystalline or UFGmicrostructure of the chip, and are therefore expected to exhibitenhanced mechanical, structural, and/or physical properties with thecapability of improved performance, especially in applications whereimproved strength-to-weight and wear resistance are critical. Examplesinclude micro- and meso-scale parts such as discs, gears, andserpentines. FIG. 10 shows disc-shaped parts that were punched fromnanocrystalline bulk tantalum and AI6061-T6 foils. FIGS. 11 and 12 showsample micro/meso-scale components that were machined fromnanocrystalline bulk Inconel 718 foils using micro-EDM. As evident fromFIGS. 10 through 12, components having maximum dimensions of up to a fewmillimeters can be created directly from the bulk form of macro-scalechips produced by this invention using scaled conventional deformationprocesses such as stamping, punching, forging, etc. (FIG. 10) and usingmaterial removal processes such as EDM, cutting, and milling (FIGS. 11and 12). Because forming and machining processes suitable for use inthis step cause negligible heating, the nanocrystalline or UFGmicrostructure of the chip can be retained.

While the invention has been described in terms of particularembodiments, it is apparent that other forms could be adopted by oneskilled in the art. Accordingly, the scope of the invention is to belimited only by the following claims.

1. A method of producing a nanocrystalline monolithic article, themethod comprising the steps of: machining a body to produce apolycrystalline chip having a nanocrystalline microstructure; andforming the monolithic article entirely from the chip by performing atleast one operation on the chip chosen from the group consisting ofdeforming the chip or removing material from the chip.
 2. A methodaccording to claim 1, further comprising the step of flattening orstraightening the chip before the step of forming the monolithicarticle.
 3. A method according to claim 1, wherein monolithic article isformed by deforming the chip.
 4. A method according to claim 3, whereinthe deforming step is chosen from the group consisting of stamping,punching and forging.
 5. A method according to claim 1, whereinmonolithic article is formed by removing material from the chip.
 6. Amethod according to claim 5, wherein the removing step is a machiningoperation chosen from the group consisting of energy beam machining,micro-milling, micro-turning, laser ablation, and micro electrodischarge machining.
 7. A method according to claim 1, furthercomprising at least one additive material process performed on themonolithic article after the forming step.
 8. A method according toclaim 1, further comprising at least one thermal treatment of themonolithic article after the forming step.
 9. A method according toclaim 8, wherein the at least one thermal treatment comprises athermo-mechanical treatment.
 10. A method according to claim 8, whereinthe thermal treatment causes recrystallization of the monolithicarticle.
 11. A method according to claim 1, wherein the body is formedof a material chosen from the group consisting of metallic,intermetallic, and ceramic materials, such that the monolithic articleis entirely formed of the material.
 12. A method according to claim 1,wherein the chip has a maximum grain size in a range of about 50 toabout 1000 nm.
 13. A method according to claim 1, wherein the monolithicarticle has a maximum grain size in a range of about 50 to about 1000nm.
 14. A method according to claim 1, wherein the chip has a maximumgrain size in a range of about 50 to about 1000 nm, and the grain sizeof the chip is substantially retained in the monolithic articlefollowing the forming step.
 15. A method according to claim 1, whereinthe chip has a minimum dimension of less than centimeter.
 16. A methodaccording to claim 1, wherein the monolithic article has a maximumdimension of less than one centimeter.
 17. A method according to claim1, wherein the monolithic article has a maximum dimension of less thanone millimeter.
 18. A method according to claim 1, wherein monolithicarticle is chosen from the group consisting of shafts, disks, and gears.19. A method according to claim 1, wherein the chip undergoes straindeformation during the machining step characterized by a shear strain ofgreater than two.
 20. A method of producing a nanocrystalline monolithicarticle, the method comprising the steps of: machining a body with amicrostructure essentially free of nano-crystals to produce apolycrystalline chip that is at least micrometer-sized and has ananocrystalline microstructure characterized by a maximum grain size ina range of about 50 to about 1000 nm as a result of a sufficient straindeformation during machining; flattening and straightening the chip; andthen forming the monolithic article entirely from the chip by performingat least one operation on the chip chosen from the group consisting ofdeforming the chip or removing material from the chip, the grain size ofthe chip being substantially retained in the monolithic article.